WARNING: The following contains spoilers for Demon Slayer: Kimetsu no Yaiba Season 2, Episode 15, "Gathering," now streaming on Funimation, Hulu and Crunchyroll.
Tengen Uzui received multiple waves of backup during his fight against Daki and Gyutaro in Demon Slayer: Kimetsu no Yaiba's "Entertainment District" arc. After the boys rushed to his aid at the battle's onset, one of his wives, Hinatsuru, joined the fray while Makio and Suma worked elsewhere in the city. The battle against the demon siblings who share the title of Upper Six has become an all-out brawl on the streets of the Entertainment District. With Tengen poisoned and Tanjiro on the brink of collapse, Hinatsuru might have just created the opening they needed to defeat Gyutaro once and for all.
Hinatsuru, Makio and Suma have had the worst go of things in the "Entertainment District" arc. Going undercover as prostitutes in the first place probably wasn't very fun, much less how quickly they were snatched up by the demon Tengen had sent them in to find. While Makio and Suma were trapped inside of Daki's belts, Hinatsuru went as far as poisoning herself in an attempt to escape before being caught by Daki. If Tengen had not found Hinatsuru when he did, she probably would have died from the poison.
When Tengen Uzui first fled his clan with his wives, he instructed each of them to always place their own lives above all else, even his own and the lives of the innocents. Turns out, all three of these fierce women have summarily ignored his direct order and stayed in the city to help in what ways they can. Makio and Suma have taken to gathering the wounded and organizing an evacuation, clearing the field of possible collateral damage -- almost as though they can feel how distraught Tengen is over the lives that have already been lost on his watch.
Hinatsuru, recovering from poison and barely able to walk, found her way to the battlefield where she unleashed a barrage of Kunai -- shinobi weapons tipped in Wisteria poison -- at Gyutaro, getting the drop on the Upper-Rank Six demon. Tengen had the wherewithal to ignore her defiance of his wishes by being there at all and lopped off Gyutaro's legs, giving Tanjiro an opening to attempt to behead him. Hinatsuru's incredible strength of will to drag her large weapon up onto a roof when she could hardly stand made it clearer than ever that Tengen's wives are fighters to the core.
Hinatsuru, Makio and Suma may even be honorary, if not official, members of the Demon Slayer Corps, given their presence in a flashback to a conversation between Ubuyashiki and Tengen. Makio and Suma have shown off their own combat skills in Daki's food locker, so it seems a reasonable assumption that once the area is clear of civilians, Tengen's remaining wives will head for the battlefield. Although Daki and Gyutaro share the title of Upper Six, a seven-on-two battle against highly skilled Demon Slayers and shinobi may be more than the sibling demons can handle.
Gyutaro has already shown previously in Demon Slayer that he can be tripped up by the shinobi's sneaky tactics, so Makio and Suma may even have more tricks up their sleeves to help combat him once they arrive. Given Gyutaro's disdain for Tengen's numerous wives, seeing them in person may be additionally frustrating for the weirdo demon, which could make him even more furiously powerful or distract him into his own demise. Tengen and Tanjiro are both in serious danger in their fight against Gyutaro, and Tengen's wives might just be the key to bringing down Upper Six before his Hashira kill count climbs to 16.
